Aqueous-based electric double-layer capacitor

ABSTRACT

An electric double-layer capacitor (EDLC) and method for manufacturing thereof. The ELDC includes at least one capacitor cell with two parallel current collectors, two opposite polarity electrodes, a separator, and a rigid dielectric frame. Each electrode is disposed on a respective current collector and impregnated with aqueous electrolyte. The frame is disposed along the perimeter on the surface of a current collector and enclosing the electrodes. The two electrodes of an individual cell are configured asymmetrically, such as being composed of different materials, having different weights, and/or having different thicknesses. The electrode material may include: activated carbon, a transitional metal oxide, a conductive polymer, and/or graphene.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part (CIP) of U.S. patentapplication Ser. No. 14/386,114 (presently allowed), which is a USnational stage entry of PCT Patent Application PCT/IL2013/050233 filedon Mar. 3, 2013, which in turn claims the benefit of priority fromIsrael Patent Publication No. 218,691. Each of the foregoing patentapplications is incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSED TECHNIQUE

The disclosed technique generally relates to capacitors, and moreparticularly, to electric double-layer capacitors.

BACKGROUND OF THE DISCLOSED TECHNIQUE

An electric double-layer capacitor (EDLC), also known as a“supercapacitor” or “ultracapacitor”, is a type of electrochemicalcapacitor, which is characterized by a very high energy density relativeto conventional capacitors. Instead of two metal plates separated by aregular dielectric material, an EDLC involves the separation of chargesin a double electric field formed at the interface between anelectrolyte and a high surface area conductor. A basic EDLC cellconfiguration is a pair of highly porous electrodes, typically includingactivated carbon, disposed on opposite faces of parallel conductiveplates known as current collectors. The electrodes are impregnated withan electrolyte, and separated by a separator consisting of a porouselectrically-insulating and ion-permeable membrane. When a voltage isapplied between the electrodes, negative ions from the electrolyte flowto the positive electrode while positive ions from the electrolyte flowto the negative electrode, such that an electric double layer is formedat each electrode/electrolyte interface by the accumulated ioniccharges. As a result, energy is stored by the separation of positive andnegative charges at each interface. The separator prevents electricalcontact between the conductive electrodes but allows the exchange ofions. When the EDLC is discharged, such as by powering an externalelectrical device, the voltage across the electrodes results in currentflow as the ions discharge from the electrode surfaces. The EDLC may berecharged and discharged again over multiple charge cycles.

The extremely high surface area of the activated carbon electrodes,combined with a separation distance between electric double layers onthe order of nanometers (compared with millimeters for electrostaticcapacitors and micrometers for electrolytic capacitors), enables theabsorption of a large number of ions per unit mass and, thus, an energydensity that is orders of magnitude greater than that of conventionalcapacitors. The electrolyte may be an aqueous-based solution (e.g., awater solution of potassium hydroxide (KOH) or sulfuric acid (H₂SO₄)) ororganic-based (e.g., acetonitrile (CH₃CN), polypropylene carbonate). Inan aqueous-based electrolyte, the voltage is limited to approximately 1V(above which water decomposes), whereas organic-based electrolytes havea higher maximum voltage of about 2.5-3.0V. Since each individual EDLCcell is limited to a relatively low voltage, multiple EDLC cells may beconnected in series to enable higher voltage operation. However, serialconnection reduces the total capacitance and also requiresvoltage-balancing.

While the amount of energy stored per unit weight is generally lower inan EDLC in comparison to electrochemical batteries, the EDLC has a muchgreater power density and a high charge/discharge rate. Furthermore, anEDLC has a far longer lifespan than a battery and can undergo many morecharge cycles with little degradation (millions of charge cycles,compared to hundreds for common rechargeable batteries). Consequently,EDLCs are ideal for applications that require frequent and rapid powerdelivery, such as hybrid vehicles that are constantly braking andaccelerating, while batteries are used to supply a larger amount ofenergy over a longer period of time. EDLCs are also environmentallyfriendly (have a long lifespan and are recyclable), safe (no corrosiveelectrolytes and other toxic materials requiring safe disposal),lightweight, and have a very low internal resistance (ESR). The chargingprocess of an EDLC is also relative simple, as it draws only is therequired amount and is not subject to overcharging. An EDLC has a higherself-discharge compared to other capacitors and electrochemicalbatteries.

During EDLC operation at high operating temperatures and/or highoperating voltages, various potentially detrimental parasitic effectstend to occur. In particular, electrochemical reactions cause excessivepressures in the electrode composition, resulting in the discharge ofgases. The built up pressures from the discharged gases could result inswelling or bursting of the capacitor elements.

Advances in materials and manufacturing methods in recent years have ledto improved performance and lower cost of EDLCs, and to theirutilization in various applications. For example, EDLCs can be employedto operate low-power electrical equipment, and to provide peak-loadenhancement for hybrid or fuel-cell vehicles. EDLCs are also commonlyused to complement batteries, such as in order to bridge short powerinterruptions in an uninterruptible power supply.

U.S. Pat. No. 4,697,224 to Watanabe et al, entitled “Electric doublelayer capacitor”, is directed to an EDLC which includes an electricallyinsulative and ion-permeable separator, and a pair of polarizableelectrodes of solid carbonaceous material which are disposed oppositeeach other on opposite sides of the separator. The separator andelectrodes are sealed within a gasket of insulating rubber. Theseparator and at least one of the electrodes are adhered to each otherby an adhesive or cohesive agent in part of a region in which theelectrode faces the separator, in order to prevent possible displacementof the electrodes and shorting via mutual contact.

European Patent No. 786,786 to Varakin, entitled “Capacitor with adouble electrical layer”, discloses an EDLC with one electrode made ofnickel oxide and the other electrode made of a fibrous carbonicmaterial, preferably nickel-plated or copper-plated. The electrolyte isan aqueous solution of an alkali metal carbonate or hydroxide.

U.S. Pat. No. 6,201,685 to Jerabek et al, entitled “Ultracapacitorcurrent collector”, discloses a nonaqueous ultracapacitor with currentcollectors comprising a conductive metal substrate, such as aluminum,which is coated with a nitride, carbide or boride of a refractory metal.The coating is intended to prevent the formation and thickening of ahighly resistive aluminum oxide layer on the current collector.

U.S. Pat. No. 6,594,138 to Belyakov et al, entitled “Electrochemicalcapacitor and method for making the same”, is directed to anelectrochemical capacitor with a bank of elements made up ofseries-connected internal elements and end elements. Each internalelement includes an electron-conducting collector, porousdifferent-polarity electrodes disposed on opposite sides of thecollector, and electron-insulating separators mounted on the electrodes.Each end element includes a collector and an electrode of an appropriatepolarity disposed on one of its sides. The electrodes and separators areimpregnated with an electrolyte. The solid phase-to-liquid ratios of theelectrodes are selected to lower the probability of electrolyte leakageduring assembly and to minimize internal resistance of the capacitor.The capacitor body includes interconnected hold-downs with electronconductors for levelling-out the voltage in the series-connectedelements. A polymeric coating is applied onto the conductors, to preventshort-circuiting of nearby elements to the electrolyte. The bank is alsocoated with a polymeric composition for sealing the elements, where thecoating includes an additional layer that eliminates the effect of theneutralizing component on the rate of polymeric hardening. The bank isevacuated at a residual pressure of 9.8-19.6 kPa prior to mountingbetween the hold-downs, enabling removal of excess air dissolved in theelectrolyte during colloidal milling of the electrode mass.

U.S. Pat. No. 6,773,468 to Lang, entitled “Method of makingelectrochemical capacitor using a printable composition”, is directed toa preparation method for an electrochemical capacitor cell thatincludes: a pair of current collector plates placed in parallel; flatelectrodes containing aqueous electrolyte printed on opposing faces ofthe current collectors; and a separator intersposed between theelectrodes. The electrodes are printed such that a peripheral region notcovered by the electrode is defined on each of the faces of the currentcollectors. The geometric form and size of the separator is identical tothe form and size of the current collector plates. The separatorincludes a central region permeable to the electrolyte surrounded by aperipheral masked region non-permeable to the electrolyte, where thepermeable region coincides with the electrodes. A sealant is impregnatedin the pores in the peripheral region of the separator. At least onelayer of adhesive is deposited on the sealant. The electrodes arefabricated using a suitable printable composition.

SUMMARY OF THE DISCLOSED TECHNIQUE

In accordance with one aspect of the disclosed technique, there is thusprovided an electric double-layer capacitor (EDLC) that includes atleast one capacitor cell, where the capacitor cell includes two currentcollectors, two electrodes of opposite polarity, a separator, and arigid dielectric frame. The current collectors are aligned with theirfaces in parallel. The current collectors are made from a conductivematerial. Each of the electrodes is disposed on a respective one of thecurrent collectors. The electrodes are impregnated with an aqueouselectrolyte. The separator is disposed between the electrodes. Theseparator includes an inert, electrically-insulating and ion-permeablematerial. The frame is disposed along the perimeter on the surface of atleast one of the current collectors and enclosing the electrodes. Thetwo electrodes of an individual capacitor cell are configuredasymmetrically, in a manner where: the electrodes are composed ofdifferent materials; the electrodes have different weights; and/or theelectrodes have different thicknesses. The electrodes may includematerials such as: activated carbon; a transitional metal oxide; aconductive polymer; and/or graphene. The EDLC may be an EDLC stack madeup of a plurality of such capacitor cells connected in series. The EDLCmay include a channel that extends out of the EDLC. The channel mayinclude at least one mechanism configured to prevent the passage ofoxygen into the EDLC. The mechanism may be a unidirectional valvedisposed in the channel, the valve configured to enable the dischargedgases to exit the EDLC while preventing gas entry into the EDLC. Themechanism may include a tube having a length and thickness configured tolimit the passage of oxygen into the EDLC. The EDLC stack may includetwo metal plates, between which the capacitor cells are fixed andpressed together. The EDLC stack may be coated with a polymeric sealantto seal in the capacitor cells. The EDLC stack may include at least onefastening mechanism configured to hold together the plates. Thefastening mechanism may include hold-down screws, disposed through theplates at the perimeter of the plates. The fastening mechanism mayinclude straps, enclosed around both plates. The EDLC stack may includeat least one support rod, disposed against the outer surface of at leastone plate and bounded by the straps. The rod is operative to straightenthe plates and to disperse pressure evenly along the surface of theelectrodes between the plates. The EDLC stack may include at least onegraphite film, disposed between one of the plates and the adjacentcapacitor cell. The graphite film is operative to prevent electrolyticleakage while maintaining electrical conductivity.

In accordance with another aspect of the disclosed technique, there isthus provided a method for manufacturing an EDLC that includes at leastone capacitor cell. The method includes the procedures of: preparing twoelectrodes of opposite polarity, the electrodes impregnated with anaqueous electrolyte; disposing each of the electrodes onto a respectiveone of two current collectors aligned in parallel, the currentcollectors made from a conductive material; disposing a separatorbetween the electrodes, the separator including an inert,electrically-insulating and ion-permeable membrane; and disposing arigid dielectric frame along the perimeter on the surface of at leastone of the current collectors, the frame enclosing the electrodes. Thetwo electrodes of an individual capacitor cell are configuredasymmetrically, in a manner where: the electrodes are composed ofdifferent materials; the electrodes have different weights; and/or theelectrodes have different thicknesses. The electrodes may includematerials such as: activated carbon; a transitional metal oxide; aconductive polymer; and/or graphene. The procedure of preparing twoelectrodes includes the procedures of: thermally treating an electrodemixture that includes activated carbon; impregnating the electrodemixture with an electrolyte solution while subjecting the mixture to acolloidal mill; terminating the colloidal mill, producing anelectrode/electrolyte paste; and rolling the paste into a sheet with aselected thickness, and cutting the sheet into multiple electrodesections with selected dimensions. The method may further include theprocedure of connecting a plurality of the capacitor cells in series, toprepare an EDLC stack. The method may further include the procedures of:casting at least one channel that extends out of the EDLC; and providingat least one mechanism in the channel, the mechanism configured toprevent the passage of oxygen into the EDLC. The method may furtherinclude the procedure of fixing and pressing together the capacitorcells between two metal plates, where the plates are held together withat least one fastening mechanism.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed technique will be understood and appreciated more fullyfrom the following detailed description taken in conjunction with thedrawings in which:

FIG. 1A is a sectional view schematic illustration of the components ofa single EDLC cell, constructed and operative in accordance with anembodiment of the disclosed technique;

FIG. 1B is an isometric view schematic illustration of the components ofFIG. 1A relatively positioned while forming the EDLC cell;

FIG. 1C is a side view schematic illustration of the formed EDLC cell ofFIGS. 1A and 1B;

FIG. 2A is a detailed sectional view schematic illustration of anindividual film of an EDLC cell, constructed and operative in accordancewith an embodiment of the disclosed technique;

FIG. 2B is a detailed sectional view schematic illustration of anindividual film of an EDLC cell, constructed and operative in accordancewith another embodiment of the disclosed technique;

FIG. 3 is an isometric sectional view schematic illustration of an EDLCstack composed of a plurality of EDLC cells, constructed and operativein accordance with an embodiment of the disclosed technique;

FIG. 4 is an isometric view schematic illustration of an EDLC stackwhich is fastened using screws, constructed and operative in accordancewith an embodiment of the disclosed technique; and

FIG. 5 is an isometric view schematic illustration of an EDLC stackwhich is fastened using rods and straps, constructed and operative inaccordance with an embodiment of the disclosed technique.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The disclosed technique overcomes the disadvantages of the prior art byproviding a novel design and arrangement for an electric double-layercapacitor (EDLC) and a method for preparation thereof. The EDLC may beembodied by a stack of multiple capacitor cells, where each cellincludes: opposite-polarity electrodes disposed on respective currentcollector films, an aqueous electrolyte impregnated on the electrodes, aseparator, and a rigid dielectric frame along the perimeter of thecurrent collector films enclosing the electrodes. The frame may includeat least one capillary for evacuating discharged gases away from theelectrodes and out of the EDLC, preventing swelling or bursting of thecapacitor, particularly at high operating temperatures/voltages. Theframe may also include compartments that provide safe storage ofresidual electrolyte, which reduces electrolyte leakage and thepossibility of electrolytic bridging between capacitor elements. TheEDLC may further include at least one mechanism for preventing oxygenentry, such as a unidirectional valve disposed in a channel that extendsthrough the EDLC stack. The two electrodes of an individual capacitorcell may be configured asymmetrically, such as being composed ofdifferent materials, having different weights, and/or having differentthicknesses. The electrode material may include: activated carbon, atransitional metal oxide, a conductive polymer, and/or graphene.

Reference is now made to FIGS. 1A, 1B and 1C. FIG. 1A is a sectionalview schematic illustration of the components of a single EDLC cell,generally referenced 110, constructed and operative in accordance withan embodiment of the disclosed technique. FIG. 1B is an isometric viewschematic illustration of the components of FIG. 1A relativelypositioned while forming EDLC cell 110. FIG. 1C is a side view schematicillustration of the formed EDLC cell 110 of FIGS. 1A and 1B. Anexemplary EDLC cell 110 of the disclosed technique is made up of a pairof films 111 and 112, and a separator 116. Each film 111, 112 iscomposed of a current collector 122 on which there is an electrode 124impregnated with an electrolyte 125. A rigid dielectric frame 126 ispositioned over the current collector 122 bordering the electrode 124,as will be elaborated upon hereinbelow. Separator 116 is disposed inbetween films 111 and 112.

Current collector 122 is made from a conductive material, such as aconductive polymer material, in which the electrical conductivity isanisotropic, such that the conductivity perpendicular to the surface ofthe current collector sheet is greater than the conductivity along thesurface. Alternatively, current collector 122 is made from a metal orother material which is inert to electrolyte 125. Electrode 124 may becomposed of a mixture of activated carbon and various additives andnanoparticles (e.g., metal oxides and hydroxides, carbon nanotubes,graphite, conductive carbons, metal nanoparticles, and the like). Theactivated carbon is prepared from raw materials such as charcoal,carbon, and coke. Electrode 124 may alternatively be composed of othermaterials, such as a transitional metal oxide (TMO), a conductivepolymer such as polyaniline (PANI), or a graphene-based layer.Electrolyte 125 is an aqueous-based solution that includes an alkalineand/or acid and salts, such as a water solution of potassium hydroxide(KOH) or sulfuric acid (H₂SO₄). Separator 116 is an inert membrane,which is ion-permeable (i.e., allowing the exchange of ionstherethrough) and electrically-insulating (i.e., preventing the transferof electrons therethrough). Separator 116 may optionally includemultiple layers (e.g., a number of separate ion-permeable andelectrically-insulating membranes arranged successively).

The preparation process for the electrodes involves thermal treatment ofthe activated carbon, followed by simultaneously impregnating theelectrode material with the electrolyte solution while subjecting theelectrode-electrolyte mixture to a colloidal mill. The colloidal millprocess is completed, resulting in a viscous paste substance.Optionally, one or both surfaces of separator 116 is also impregnatedwith the electrolyte solution 125. The electrode/electrolyte paste isrolled into a sheet with a suitable thickness (e.g., approximately 700μm), and the sheet is precisely cut into multiple electrode sectionswith selected dimensions. An individual electrode section is thendisposed onto a current collector film.

Each EDLC cell 110 includes at least two electrodes 124A, 124B withopposite polarity, and a separator 116 in between. The application of avoltage between electrodes 124A, 124B results in electrolytic ionictransfer and the formation of an electric double-layer at each electrodeand electrolyte interface. The complementary electrodes 124 of a cell110 may be composed of the same materials (e.g., an activated carbonmixture) and thus be substantially equal in weight and thickness (i.e.,“a symmetrical electrode configuration”). Alternatively, thecomplementary electrodes 124 of a cell 110 may be composed of differentmaterials and have different weights or thicknesses (i.e., “anasymmetrical electrode configuration”). For example, an asymmetricalelectrode configuration may include an anode composed of an activatedcarbon mixture and a cathode composed of a TMO with conductive additives(at various concentrations), such as an activated carbon anode with amanganese dioxide (MnO₂) cathode at a weight ratio of approximately1:0.8 (C:MnO₂). An asymmetrical configuration may also includecomplementary electrodes of the same material but with differentweights/thicknesses, such as an activated carbon anode with an activatedcarbon cathode at a thickness ratio of approximately 1:1.25. Anasymmetrical electrode configuration may provide increased cell voltage,increased capacitance, and improved performance, especially at hightemperatures.

Multiple cells are arranged on top of one another and connected inseries to form a stack, providing an EDLC which is able to withstand ahigher voltage (compared to that of an individual EDLC cell). Referenceis now made to FIGS. 2A, 2B and 3. FIG. 2A is a detailed sectional viewschematic illustration of an individual film 111 of the EDLC cell 110,constructed and operative in accordance with an embodiment of thedisclosed technique. FIG. 2B is a detailed sectional view schematicillustration of an individual film, referenced 211, of an EDLC cell 110,constructed and operative in accordance with another embodiment of thedisclosed technique. FIG. 3 is an isometric sectional view schematicillustration of an EDLC stack, generally referenced 150, composed of aplurality of EDLC cells 110, constructed and operative in accordancewith an embodiment of the disclosed technique. Film 111 includes acurrent collector 122, an electrode 124, and a frame 126. Referring toFIG. 2A, frame 126 includes a plurality of compartments 131, at leastone capillary 132, and at least one notch 133. Hollow channel 134 issituated through EDLC stack 150 extending across all of the cells 110(i.e., into the cross-sectional plane defined by each EDLC cell) andterminating in a one-way valve 148 (depicted in FIGS. 4 and 5). Stack150 includes a first external terminal lead-out 142 and a secondexternal terminal lead-out 144 (depicted in FIGS. 4 and 5). All thecells of a stack are fixed between two metal plates 146 and pressedtogether at a sufficient pressure (e.g., 10 kgF/cm²) in accordance withthe size and dimensions of the EDLC cells 110. The two plates 146 areheld together via at least one fastening mechanism, such as hold-downscrews 138 (FIG. 4) or rods 152 and straps 154 (FIG. 5). Plates 146 aremade from a metal that is inert to electrolyte 125. Stack 150 is coatedwith a polymeric sealant 139 to seal in all of the cells 110. During thesealing process, two hollow channels 134 are casted, such that thecapillaries 132 protruding from each cell are assembled within theinterior of the channels 134, ensuring that the capillaries are notfilled with the sealant 139. Stack 150 may alternatively include asingle channel 134 or more than two channels 134. Channels 134 may beembodied by a discrete tube which passes through the casted regionwithin stack 150 and exits out of stack 150. The negative electrodes124A of all EDLC cells 110 are electrically coupled with first terminallead-out 142, and the positive electrodes 124B of all EDLC cells 110 areelectrically coupled with second terminal lead-out 144. The externalends of first terminal lead-out 142 and second terminal lead-out 144 areelectrically connected to respective terminals of an external powersource.

Frame 126 is a rigid border which is disposed over current collector 122on the edge of film 111 at the periphery of electrode 124. Frame 126 ismade of a dielectric material that is inert to the aqueous electrolyte125. Possible materials for frame 126 include: polyvinyl chloride (PVC),polypropylene (PP), polytetrafluoroethylene (PTFE) aka Teflon, EPDMrubber, and other polymers. Both surfaces of film 111 (i.e., both thefront and rear) includes a respective frame 126. Frame 126 is adheredonto current collector 122 using a suitable adhesive material and/oradhesion technique (e.g., glue, heating, laser, soldering, and thelike). Separator 116 is, in turn, adhered onto frame 126 via an adhesiveor sealant (while the opposite surface of separator is adhered onto therespective frame of the adjacent current collector/electrode film 112).To facilitate the fabrication process, individual layers of the filmsmay be prepared separately and then adhered to one another (e.g., oneframe 126 is adhered to a first surface of a first film 111, anotherframe 126 is adhered to a first surface of a second film 112, and thenthe second surface of film 111 is adhered to the first surface of film112). Preferably, there is an electrode 124 on each surface (i.e., oneon the front and one on the rear) of every film of EDLC stack 150 exceptfor the very first (i.e., uppermost) film and the very last (i.e.,lowermost) film.

Frame 126 serves to delimit the electrode region of the film, tofacilitate the pressing together of multiple films when preparing thestack, and to further constrict the layers in the stack to minimize theinternal resistance of the capacitor. Additionally, frame 126 isoperative to isolate between the adjacent films 111, 112 and to preventthe aqueous electrolyte 125 from leaking through the edges of the films,particularly while compressing films in the stack, thereby preventingcharge transfer between electrodes 122A and 122B which would result in ashort circuit (i.e., “electrolytic bridging”). It is appreciated thatthe frame 126 of the disclosed technique may be utilized in conjunctionwith electrodes with variable thicknesses, which makes it possible toensure reproducibility and stable electrical performance characteristicsat a variety of charging currents.

Compartments 131 (FIG. 2A) are interspersed throughout frame 126.Preferably, a compartment 131 projects slightly inwards with respect tothe flat surface of frame 126 to form an indentation or groove withinframe 126. It is noted that a compartment 131 may extend all the way tothe edge of frame 126, or may be fully situated within the framemargins. Notch 133 is formed at the inner perimeter of frame 126 andextends up to a compartment 131, such that notch 133 is connected to acompartment 131 at one end and to a current collector region 122 at theedge of electrode 124 at the other end. At least one compartment 131 inframe 126 is connected to at least one notch 133, i.e., frame 126 mayinclude some compartments 131 that are not connected to a notch 133(note that only a single notch 133 is depicted in FIG. 2A forillustrative purposes only). Capillary 132 is disposed within acompartment 131 and protrudes out of frame 126 and into channel 134.Each electrode 124 in EDLC stack 150 is associated with at least onecapillary 132 (i.e., each EDLC cell 110 is associated with at least apair of capillaries 132). All the capillaries 132 from individual cells110 in the stack 150 are assembled within channel 134. It is appreciatedthat capillaries 132 are optional, and a frame of the disclosedtechnique may alternatively include no capillaries.

Compartments 131 serve as storage regions for residual electrolyte 125that seeps out from electrodes 124 due to externally applied pressure,such as when films 111, 112 are pressed together during the fabricationprocess of EDLC cell 110. This residual electrolyte 125 is collectedwithin compartments 131, preventing leakage currents that would resultif the electrolyte 125 exits frame 126 (e.g., if stack 150 has poorsealing), and preventing electrolytic bridging between electrodes 124A,124B (as frame 126 maintains isolation of the electrolyte 125 from theelectrodes 124). In addition, the residual electrolyte 125 beingcollected within compartments 131 also serves to limit electrolyticdehydration of EDLC stack 150. It is appreciated that compartments 131are optional, and a frame of the disclosed technique may alternativelyinclude no compartments, as depicted in FIG. 2B (discussed furtherherein below).

Capillaries 132 provide an evacuation mechanism for the excess gasesreleased by electrodes 124 due to various parasitic effects during EDLCoperation, particularly at high operating temperatures and/or highoperating voltages. These gases are evacuated via capillaries 132 andnotches 133, allowing for the built-up pressures in EDLC cells 110 to bereleased, and avoiding swelling or even bursting of the capacitor insuch conductions. Capillaries 132 are made from a porous hydrophobicmaterial, such as PTFE (Teflon) or another suitable hydrophobic polymer,such that gases are capable of passing through the capillary walls butliquids cannot. Thus, capillaries 132 allow for the removal of thereleased gases while keeping inside the aqueous electrolyte 125, therebyminimizing electrolytic dehydration. Accordingly, the gases produced bythe aforementioned parasitic effects exit electrode region 124 of EDLCcell 110 through notches 133 and the walls of capillaries 132, fromwhere the gases are transported through channel 134 out of EDLC stack150 via external unidirectional valve 148. It is appreciated that therelatively small thickness of the capillary walls (e.g., approximately0.17 mm), enables efficient and rapid evacuation of the gases out fromEDLC stack via capillaries 132. The distal end of capillaries 132 (i.e.,the end protruding into channel 134) are preferably initially treatedwith a chemical composition that provides improved adhesion, ensuringthat the capillaries remain adhered to the EDLC stack 150 after thefinal sealant 139 is applied. Capillaries 132 may also be chemicallytreated in a certain way in order to prevent the escape of gas fromaround the capillary 132.

Unidirectional valve 148 ensures that external oxygen does not enterEDLC stack 150 and reach electrodes 124 by passing back through channel134 and capillaries 132 (i.e., in the reverse direction as the evacuatedgases), as the incoming oxygen could result in current leakages.Furthermore, channel 134 may include a relatively long and narrow tube(e.g., a tube having a minimal length substantially equal to the widthof stack 150 and having a minimal thickness substantially equal to thediameter of the capillaries 132), which prevents or minimizes diffusionof the excess gases and electrolyte, and which further serves to limitthe passage of oxygen back into EDLC stack 150. In general, thedisclosed technique employs at least one mechanism for preventing thepassage of oxygen into EDLC stack 150, including but not limited to, theuse of a unidirectional valve, and the use of a tube having a length andthickness that limits the passage of oxygen into said EDLC.

Referring now to FIG. 2B, film 211 includes a current collector 222, anelectrode 224A, a frame 226, and at least one capillary 232. Electrode224A has a freeform and asymmetrical shape, in contrast to thesubstantially square-shaped and symmetrical electrode 124A of FIG. 2A.It is appreciated that the freeform shape of electrode 224A provides fora larger electrode surface area, as compared to electrode 124A, for agiven sized current collector film 211, and thus provides a largermaximum energy density for the capacitor (which is proportional to theelectrode surface area). The inner edge of frame 226 is alsoasymmetrically-shaped to conform to the shape of electrode 224A.Capillary 232 passes directly through frame 226, rather than beingdisposed within a compartment situated in the frame (as with capillary132 and compartment 131 of FIG. 2A). Capillary 232 extends along aportion of electrode 224A and protrudes out of frame 226 and intochannel 234. The capillaries 232 protruding from each cell 211 in thestack 150 are assembled within the interior of channel 234, which passesthrough to the exterior of the stack 150, as with channel 134 (FIG. 3).Capillary 232 is operative for evacuating gases discharged by electrodes124 (resulting from parasitic effects) out of the EDLC stack 150 via thecapillary walls, analogous to capillary 132 (FIG. 2A). Capillary 232 iscomposed of a porous hydrophobic material (e.g., Teflon), such thatgases are capable of passing through the capillary walls but liquidscannot. It is noted that capillary 232 is optionally formed in aU-shape, which helps prevent the aqueous electrolyte 125 from exitingthe electrode 224A via the interior of capillary 232 (since thehydrophobic material of capillary only prevents liquid from passingthrough the capillary walls), thereby minimizing electrolyticdehydration. Accordingly, capillary 232 may also be tied or closed offat its end, for preventing release of aqueous electrolyte 125 via thecapillary interior.

Reference is now made to FIGS. 4 and 5. FIG. 4 is an isometric viewschematic illustration of an EDLC stack, generally referenced 160, whichis fastened using screws, constructed and operative in accordance withan embodiment of the disclosed technique. FIG. 5 is an isometric viewschematic illustration of an EDLC stack, generally referenced 170, whichis fastened using rods and straps, constructed and operative inaccordance with an embodiment of the disclosed technique. The fasteningmechanism ensures that pressure is maintained between plates 146. Thispressure also helps to overcome minor discrepancies in parallelism,thickness and other parameters of the EDLC stack, reducing internalresistance of the EDLC. Referring to FIG. 4, the outer metal plates 146of stack 160 are fastened together via a plurality of hold-down screws138, such as clamping screws, disposed through both plates 146 at theperimeter of the plates 146 and cells 110. Referring to FIG. 5, outermetal plates 146 of stack 170 are fastened together via a plurality ofstraps 154 enclosed around both plates 146 and all the cells 110. It isappreciated that the use of straps 154 is preferable to screws 138, asit reduces the overall weight of the EDLC and requires less space.Additionally, the use of hold-down screws 138 tends to cause a curvatureof plates 146 due to the force differential between the center and edgesof plates resulting from the pressure exerted by screws 138 along theperimeter of plates 146. Therefore, the use of straps 154 has the addedbenefit of avoiding such curvature. Alternatively, plates 146 may beprovided initially concave or with a predefined level of curvature inorder to compensate for this effect, such that the plates 146 eventuallystraighten out after the application of screws 138 and suitableparallelism is maintained. Rods 152 are placed against the outer surfaceof plates 146 and are bounded by straps 154. Rods 152 serve tostraighten out curvature in plates 148, and to disperse the appliedpressure evenly along the entire surface of electrodes 124, therebyminimizing internal resistance. At least one graphite layer (not shown)is disposed between the final film in the EDLC stack and the adjacentmetallic plate 146, preferably on both sides of the stack, in order toprevent electrolytic leakage while maintaining electrical conductivity.

It is appreciated than an exemplary EDLC of the disclosed techniquegenerally operates over a prolonged lifespan at substantially hightemperature levels and substantially high nominal/working voltages, incomparison to conventional organic-based capacitors. For example, anEDLC of the disclosed technique is capable of operation at a temperaturerange between approximately −40° C. and 75° C. It is further noted thatan EDLC of the disclosed technique does not require external balancing(i.e., to maintain balanced voltage across the series-connected EDLCcells), and can be considered as being “self-balanced” In particular,the EDLC of the disclosed technique has a substantially robust structureand is substantially insensitive to voltage surges. Furthermore, thevariation in capacitance (i.e., tolerance) between different cells isvery small (e.g., a few percentages, as compared to capacitancevariations of up to 30% in commercial organic capacitors). This is dueto a number of factors. Firstly, the use of relatively thick electrodes(e.g., a thickness of approximately 700 μm). Secondly, the nominalworking voltage of an individual cell in the stack is approximately 0.9V(rather than 1V, which corresponds to the electrolysis limit). Thirdly,the EDLC of the disclosed technique involves a “closed-loop”electrochemical system, since even if electrolysis occurs, thecapillaries will evacuate the water vapor and other gases and preventthe capacitor from swelling or bursting, but the electrodes willdehydrate. The electrode dehydration causes an increase of the internalresistance, and thus an increase in the voltage, of the electrodes,resulting in a lower overall voltage limit for electrolysis (therebypreventing electrolysis from occurring).

In accordance with the disclosed technique, a method for manufacturingan EDLC includes preparing at least once capacitor cell by preparing twoelectrodes of opposite polarity, the electrodes impregnated with anaqueous electrolyte; disposing each of the electrodes onto a respectiveone of two current collectors aligned in parallel, the currentcollectors including a conductive material; disposing a separatorbetween the electrodes, the separator including an inert,electrically-insulating and ion-permeable material; and disposing arigid dielectric frame along the perimeter on the surface of at leastone of the current collectors, the frame enclosing the electrodes. Thetwo electrodes of an individual cell are configured asymmetrically, suchas being composed of different materials, having different weights,and/or having different thicknesses. The electrode material may include:activated carbon, a transitional metal oxide, a conductive polymer,and/or graphene.

It will be appreciated by persons skilled in the art that the disclosedtechnique is not limited to what has been particularly shown anddescribed hereinabove.

The invention claimed is:
 1. An electric double-layer capacitor (EDLC)comprising at least one capacitor cell, said capacitor cell comprising:two current collectors, aligned with their faces in parallel, saidcurrent collectors comprising a conductive material; two electrodes ofopposite polarity, each of said electrodes disposed on a respective oneof said current collectors, said electrodes impregnated with an aqueouselectrolyte; a separator, disposed between said electrodes, saidseparator comprising an inert, electrically-insulating and ion-permeablematerial; and a rigid dielectric frame, disposed along the perimeter onthe surface of at least one of said current collectors and enclosingsaid electrodes, wherein the two electrodes comprising an individualcapacitor cell are configured asymmetrically, in a manner selected fromthe list consisting of: said electrodes being composed of differentmaterials; said electrodes having different weights; said electrodeshaving different thicknesses; and any combination of the above.
 2. TheEDLC of claim 1, wherein at least one of said electrodes comprises amaterial selected from the list consisting of: activated carbon; atransitional metal oxide; a conductive polymer; and graphene.
 3. An EDLCstack, comprising a plurality of capacitor cells as in claim 1 connectedin series.
 4. The EDLC of claim 3, wherein said stack is coated with apolymeric sealant to seal in said capacitor cells.
 5. The EDLC of claim3, comprising two metal plates, between which said capacitor cells arefixed and pressed together.
 6. The EDLC of claim 5, further comprisingat least one fastening mechanism, configured to hold together saidplates.
 7. The EDLC of claim 5, further comprising at least one graphitefilm, disposed between one of said plates and the adjacent capacitorcell, said graphite film configured to prevent electrolytic leakagewhile maintaining electrical conductivity.
 8. The EDLC of claim 1,further comprising a channel that extends out of said EDLC, wherein saidchannel comprises at least one mechanism configured to prevent thepassage of oxygen into said EDLC.
 9. The EDLC of claim 8, wherein saidmechanism comprises a unidirectional valve disposed in said channel,said valve configured to enable said discharged gases to exit said EDLCwhile preventing gas entry into said EDLC.
 10. The EDLC of claim 8,wherein said mechanism comprises a tube having a length and thicknessconfigured to limit the passage of oxygen into said EDLC.
 11. A methodfor manufacturing an electric double-layer capacitor (EDLC) comprisingat least one capacitor cell, the method comprising the procedure of:preparing two electrodes of opposite polarity, said electrodesimpregnated with an aqueous electrolyte; disposing each of saidelectrodes onto a respective one of two current collectors aligned inparallel, said current collectors comprising a conductive material;disposing a separator between said electrodes, said separator comprisingan inert, electrically-insulating and ion-permeable membrane; anddisposing a rigid dielectric frame along the perimeter on the surface ofat least one of said current collectors, said frame enclosing saidelectrodes, wherein the two electrodes comprising an individual cell areconfigured asymmetrically, in a manner selected from the list consistingof: said electrodes being composed of different materials; saidelectrodes having different weights; said electrodes having differentthicknesses; and any combination of the above.
 12. The method of claim11, wherein at least one of said electrodes comprises a materialselected from the list consisting of: activated carbon; a transitionalmetal oxide; a conductive polymer; and graphene.
 13. The method of claim11, wherein said procedure of preparing two electrodes comprises theprocedures of: thermally treating an electrode mixture comprisingactivated carbon; impregnating said electrode mixture with anelectrolyte solution while subjecting said mixture to a colloidal mill;terminating said colloidal mill, producing an electrode/electrolytepaste; and rolling said paste into a sheet with a selected thickness,and cutting the sheet into multiple electrode sections with selecteddimensions.
 14. The method of claim 11, further comprising the procedureof connecting a plurality of said capacitor cells in series, to preparean EDLC stack.
 15. The method of claim 11, further comprising theprocedures of: casting at least one channel that extends out of saidEDLC; and providing at least one mechanism in said channel, saidmechanism configured to prevent the passage of oxygen into said EDLC.16. The method of claim 14, further comprising the procedure of fixingand pressing together said capacitor cells between two metal plates,where said plates are held together with at least one fasteningmechanism.